ASTM D6602 “Standard Practice for Sampling and Testing of Possible Carbon Black Fugitive Emissions or Other Environmental Particulate, or Both” was developed for the investigation of outdoor sooty surface problems and distinguishing manufactured carbon black, in the N100 to N900 series, from other environmental particulates. It is a microscopy technique that is well suited for the study of darkening agent complaints. Samples are collected from surfaces with cotton balls, wipes, and tape-lifts. The particulate is then analyzed by light microscopy and electron microscopy. The practice is used as a means to differentiate between darkening caused by fungal growth (biofilms), soots, or other dark particles. Polarized light microscopy provides information about the relative amounts of the different types of particles present and some information about the type of biofilm, if present. Transmission electron microscopy provides confirmation of aciniform soot, if present, and some information about the possible source of the soot. An investigation of dark surface particulate in a city where both a carbon black manufacturing facility and an oil refinery were possible industrial sources provides an illustration of the use of the method.